1. Field of the Invention
The present invention relates to a rare earth oxide type phosphor which is excited by vacuum ultraviolet rays (VUV) having a wavelength of at most 200 nm and emits light highly efficiently, a phosphor paste containing said phosphor, and a light-emitting device comprising a phosphor layer containing said phosphor. Particularly, it relates to a rare earth oxide type phosphor for VUV excitation, with which a decrease of brightness with time is small, suitable for a vacuum ultraviolet ray excitation type light-emitting device (VUV excitation type light-emitting device) such as a plasma display panel (PDP) or a rare gas discharge lamp.
2. Discussion of Background
A VUV excitation type light-emitting device represented by e.g. PDP or a rare gas discharge lamp, comprises a transparent envelope and a phosphor layer containing a phosphor for VUV formed inside of the envelope, so that the phosphor layer is excited by VUV having a wavelength of at most 200 nm, radiated by discharge of a rare gas sealed in the envelope, such as Ar, Xe, He, Ne, Hexe2x80x94Xe or Nexe2x80x94Xe, to emit visible light.
With respect to the phosphor for VUV to be used for the phosphor layer for the VUV excitation type light-emitting device, it undergoes baking deterioration due to heat in a baking treatment at the time of the phosphor layer formation, and further, it undergoes deterioration due to VUV irradiation or ion impact at the time of discharge of a rare gas, thus leading to a decrease of luminescence brightness. Accordingly, it is required for the phosphor for VUV that the baking deterioration is small (baking resistance) and the deterioration due to VUV or ion impact is small (VUV resistance, ion impact resistance).
Here, as the phosphor for VUV, a red color-emitting phosphor such as (Y,Gd)BO3:Eu, Y2O3:Eu or (Y,Gd)2O3:Eu, a green color-emitting phosphor such as Zn2SiO4:Mn, LaPO4:Ce, Tb or (Ba, Sr, Mg)O.aAl2O3:Mn, or a blue color or blue green color-emitting phosphor such as BaMgAl10O17:Eu, BaMgAl10O17:Eu or Mn, may, for example, be used.
Among these phosphors for VUV, particularly with respect to a rare earth oxide type phosphor for VUV excitation represented by the formula (Y1-x-y, Lnx, Rey)2O3 wherein Ln is at least one rare earth element selected from the group consisting of Gd, La and Lu, Re is at least one rare earth element selected from the group consisting of Eu, Tm, Sm and Pr, and x and y are numbers which satisfy conditions 0xe2x89xa6xxe2x89xa61xe2x88x92y and 0.001xe2x89xa6yxe2x89xa60.1, such as Y2O3:Eu or (Y,Gd)2O3:Eu, when continuously used as a phosphor layer for a VUV excitation type light-emitting device, a decrease of brightness with time tends to be significant, and improvement has been desired.
The present invention has been made to overcome the above problems, and it is an object of the present invention to provide a rare earth oxide type phosphor with which a decrease of luminescence brightness with time is small, and which is excellent in VUV resistance with less deterioration due to VUV or ion impact, when used under excitation by VUV having a wavelength of at most 200 nm for a long period of time, a phosphor paste composition and a VUV excitation type light-emitting device.
The present inventors have conducted extensive studies on improvement of the VUV resistance of a phosphor by subjecting the phosphor to surface treatment, and as a result, have found that by coating the surface of a rare earth oxide type phosphor with a compound containing Mg and P in its composition such as magnesium phosphate, the VUV resistance improves, and the decrease of luminescence brightness with time can be suppressed when the phosphor is irradiated with VUV to emit light, and the present invention has been accomplished.
Namely, the present invention provides:
(1) A phosphor for vacuum ultraviolet ray excitation, which comprises a rare earth oxide type phosphor, wherein at least part of the surface of the rare earth oxide type phosphor is coated with a compound containing at least a magnesium element and a phosphorus element.
(2) The phosphor for vacuum ultraviolet ray excitation according to the above (1), wherein the vacuum ultraviolet rays for excitation have a wavelength of at most 160 nm.
(3) The phosphor for vacuum ultraviolet ray excitation according to the above (1) and (2), wherein the rare earth oxide type phosphor is represented by the following formula:
(Y1-x-y, Lnx, Rey)2O3 
wherein Ln is at lease one rare earth element selected from the group consisting of Gd, La and Lu, Re is at least one rare earth element selected from the group consisting of Eu, Tm, Sm and Pr, and x and y are numbers which satisfy conditions 0xe2x89xa6xxe2x89xa61xe2x88x92y and 0.001xe2x89xa6yxe2x89xa60.2.
(4) The phosphor for vacuum ultraviolet ray excitation according to the above (3), wherein Re is Eu.
(5) The phosphor for vacuum ultraviolet ray excitation according to any one of the above (1) to (4), wherein the compound containing at least a magnesium element and a phosphorus element is a magnesium phosphate type compound.
(6) The phosphor for vacuum ultraviolet ray excitation according to any one of the above (1) to (5), wherein the content of magnesium is within a range of from 0.001 to 2 wt %, preferably from 0.01 to 0.5 wt %, on the basis of the weight of the rare earth oxide type phosphor.
(7) The phosphor for vacuum ultraviolet ray excitation according to any one of the above (1) to (6), wherein the content of phosphorus is within a range of from 0.001 to 2 wt %, preferably from 0.01 to 0.4 wt %, on the basis of the weight of the rare earth oxide type phosphor.
(8) A phosphor paste composition comprising a binder solution and the phosphor for vacuum ultraviolet ray excitation as defined in any one of the above (1) to (7), dispersed in the solution.
(9) A vacuum ultraviolet ray excitation type light-emitting device comprising an envelope and a phosphor layer formed inside of the envelope, so that the phosphor layer is excited by vacuum ultraviolet rays generated by discharge of a rare gas sealed in the envelope, to emit light, wherein the phosphor layer comprises the phosphor for vacuum ultraviolet ray excitation as defined in any one of the above (1) to (7).
(10) The vacuum ultraviolet ray excitation type light-emitting device according to the above (9), wherein the vacuum ultraviolet rays include at least vacuum ultraviolet rays having wavelengths of from 160 to 120 nm.
According to the present invention, there is provided a rare earth oxide type phosphor which highly efficiently emits light under excitation with vacuum ultraviolet rays (VUV) having a wavelength of at most 200 nm, particularly at most 160 nm, and which has an excellent brightness maintenance ratio, by coating at least part of the surface of a rare earth oxide type phosphor with a magnesium phosphate type compound containing at least a Mg element and a P element as constituents in its composition, such as magnesium phosphate.
The rare earth oxide type phosphor for VUV excitation may be a phosphor which emits light with high brightness under irradiation with VUV having a wavelength of at most 200 nm, and a phosphor represented by the following formula may, for example, be used:
(Y1-x-y, Lnx, Rey)2O3 
wherein Ln is at least one rare earth element selected from the group consisting of Gd, La and Lu, Re is at least one rare earth element selected from the group consisting of Eu, Tm, Sm and Pr, and x and y are numbers which satisfy conditions 0xe2x89xa6xxe2x89xa61xe2x88x92y and 0.001xe2x89xa6yxe2x89xa60.2, preferably 0xe2x89xa6xxe2x89xa60.4xe2x88x92y and 0.02xe2x89xa6yxe2x89xa60.1. Specifically, a Y2O3:Eu phosphor and (Y,Gd)2O3:Eu phosphor may, for example, be mentioned.
The magnesium phosphate type compound to be used for coating of the surface of the rare earth oxide type phosphor of the present invention is not particularly limited so long as it is a compound containing at least a Mg element and a P element in its composition, and examples of which include orthohydrogen phosphates of Mg such as magnesium orthophosphate (Mg3(PO4)2), magnesium hydrogenphosphate (MgHPO4) and magnesium dihydrogenphosphate (Mg(H2PO4)2), magnesium metaphosphates such as magnesium dimetaphosphate (MgP2O6), magnesium trimetaphosphate (Mg3(P3O9)2) and magnesium tetrametaphosphate (Mg2P4O12), and magnesium salts containing chain ions [(PO3)nO](n+2)(nxe2x89xa72) formed by condensation of magnesium phosphate and phosphate ions, such as magnesium polyphosphates such as Mg2P2O7 and Mg3P2O13, and hydrates thereof.
In the present invention, the contents of Mg and P in the coating on the surface of the phosphor vary depending upon the chemical composition of the magnesium phosphate type compound, but the Mg content is from 0.001 to 2 wt %, preferably from 0.01 to 0.5 wt %, based on the weight of the phosphor, and the content of P is from 0.001 to 2 wt %, preferably from 0.01 to 0.4 wt %, based on the weight of the phosphor. If the content of Mg is less than 0.001 wt % or the content of P is less than 0.001 wt %, the decrease of luminescence brightness with time of the phosphor to be obtained due to irradiation with VUV is not substantially suppressed, and if the content of Mg is higher than 2 wt % or the content of P is higher than 2 wt %, the decrease of the initial brightness of the phosphor to be obtained tends to be significant, such being unfavorable.
In the present invention, as a method of coating the surface of the rare earth oxide type phosphor with the magnesium phosphate type compound, for example, the rare earth oxide type phosphor is suspended in e.g. water to prepare a phosphor slurry, and a soluble Mg compound such as magnesium sulfate is added thereto, or a solution having a Mg compound dissolved therein is prepared separately, and this solution is added to the phosphor slurry so that Mg2+ ions coexist in the phosphor slurry. Then, the phosphorus compound containing phosphate ion, such as ammonium phosphate or ammonium polyphosphate is added to the phosphor slurry and made to react with Mg2+ to form an insoluble magnesium phosphate type compound so that at least part of the surface of the phosphor particles is coated with the magnesium phosphate type compound. Then, dehydration and drying at from 100 to 200xc2x0 C. are carried out to form a coating of the magnesium phosphate type compound on at least part of the surface of the phosphor to obtain the phosphor of the present invention. Here, with respect to the order of addition of the Mg compound and the phosphorus compound to be added to the phosphor slurry, the Mg compound is added first in the above explanation, however, needless to say, the phosphorus compound may be added first to conduct coating similarly.
The rare earth oxide type phosphor coated with a magnesium phosphate type compound thus obtained may further be baked at a temperature of from 200 to 800xc2x0 C., in order to reinforce the coating force of the magnesium phosphate type compound, to remove water content in the coating or to change e.g. hydroxyl groups into an oxide.
The phosphor paste composition for VUV of the present invention is obtained by kneading the above phosphor and an organic solvent having a predetermined amount of a binder dissolved therein (binder-containing solution) so that the phosphor and the binder are uniformly dispersed in the organic solvent. Here, the amount of the organic solvent in the binder-containing solution may optionally be increased or decreased to obtain a desired viscosity. Further, an organic solvent may further be added to the prepared phosphor paste composition so as to adjust the viscosity of the phosphor paste composition.
As the binder, a resin such as ethyl cellulose, an acryl resin, polystyrene oxide or nitrocellulose may be used, and as the organic solvent, butyl carbitol, butyl carbitol acetate, terpineol, butyl acetate, ethyl acetate or methyl ethyl ketone may, for example, be used.
The amount of the phosphor in the phosphor paste composition of the present invention is suitably from 5 to 80 wt %, preferably from 20 to 60 wt %, based on the total amount of the paste. Further, the amount of the binder in the binder-containing solution is suitably from 2 to 80 wt %, preferably from 5 to 40 wt %, based on the binder-containing solution.
The VUV light-emitting device of the present invention can be produced by a conventional method by using the above phosphor paste composition. Namely, when the VUV light-emitting device of the present invention is PDP, it is produced in such a manner that, for example, by using a glass plate having plurality of partition walls (ribs) and electrodes provided thereon, the phosphor paste composition of the present invention is coated on the inner surface of plural recesses surrounded by the partition walls (ribs), dried and then baked at a temperature of from about 300 to about 550xc2x0 C. to prepare a rear plate having a phosphor layer and electrodes formed thereon, and this rear plate and a front plate made of e.g. a flat glass plate which is transparent to visible light, having e.g. an electrode, a dielectric layer and a protective layer laminated in order thereon, are disposed to face each other, the periphery of the front and rear plates is sealed to form an envelope, the interior of the envelope is evacuated, and a rare gas such as Xe, Ar, He or Ne by itself or in combination as mixed is sealed in.
Further, when the VUV light-emitting device of the present invention is a rare gas lamp, it is produced in such a manner that, for example, the above phosphor paste composition comprising the phosphor for VUV of the present invention is coated on an inner wall of a glass tube having an inner diameter of from 4 to 12 mm, dried at from 100 to 200xc2x0 C. and then baked at from 400 to 800xc2x0 C. for from 5 to 30 minutes, and electrodes of e.g. nickel are attached to both ends of the glass tube, to the inside and outside of the glass tube or to the outer surface of the glass tube, the interior of the tube is evacuated, a rare gas such as a mixed gas of Nexe2x80x94Xe or a mixed gas of Hexe2x80x94Xe is sealed in, and both ends of the glass tube are sealed.
Now, the present invention will be described in detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples.
Preparation of Phosphor